<!DOCTYPE html>
<html lang="en-US">
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width,initial-scale=1">
    <title>EventLoop | 星月前端博客</title>
    <meta name="generator" content="VuePress 1.8.2">
    <link rel="icon" href="/favicon.ico">
    <meta name="description" content="The top of the mountain is not the end, nor is the other mountain.">
    <meta name="viewport" content="width=device-width,initial-scale=1,user-scalable=no">
    
    <link rel="preload" href="/assets/css/0.styles.f057962c.css" as="style"><link rel="preload" href="/assets/js/app.b0d6e6c9.js" as="script"><link rel="preload" href="/assets/js/3.3f705cf6.js" as="script"><link rel="preload" href="/assets/js/1.5432b32a.js" as="script"><link rel="preload" href="/assets/js/14.e5bc98a2.js" as="script"><link rel="prefetch" href="/assets/js/10.a8f43ca8.js"><link rel="prefetch" href="/assets/js/11.f3a618ea.js"><link rel="prefetch" href="/assets/js/12.b0d83b4c.js"><link rel="prefetch" href="/assets/js/13.f1fb4696.js"><link rel="prefetch" href="/assets/js/15.50519e37.js"><link rel="prefetch" href="/assets/js/16.3c6a720c.js"><link rel="prefetch" href="/assets/js/17.39fb3a22.js"><link rel="prefetch" href="/assets/js/18.76eb3752.js"><link rel="prefetch" href="/assets/js/19.7e9cf824.js"><link rel="prefetch" href="/assets/js/20.91bdeca4.js"><link rel="prefetch" href="/assets/js/21.136cc362.js"><link rel="prefetch" href="/assets/js/22.ec4f40f8.js"><link rel="prefetch" href="/assets/js/23.50127d3e.js"><link rel="prefetch" href="/assets/js/24.793b4c66.js"><link rel="prefetch" href="/assets/js/25.e2402b1b.js"><link rel="prefetch" href="/assets/js/26.cda4008e.js"><link rel="prefetch" href="/assets/js/27.8f1f302e.js"><link rel="prefetch" href="/assets/js/28.15cd65f2.js"><link rel="prefetch" href="/assets/js/29.00adce9e.js"><link rel="prefetch" href="/assets/js/30.c29d2b5d.js"><link rel="prefetch" href="/assets/js/31.40db97f7.js"><link rel="prefetch" href="/assets/js/32.70ed39cf.js"><link rel="prefetch" href="/assets/js/33.b0f7cfc3.js"><link rel="prefetch" href="/assets/js/34.f0a63370.js"><link rel="prefetch" href="/assets/js/35.88e20dad.js"><link rel="prefetch" href="/assets/js/36.13f74559.js"><link rel="prefetch" href="/assets/js/37.1d442fc4.js"><link rel="prefetch" href="/assets/js/38.84077197.js"><link rel="prefetch" href="/assets/js/39.51512938.js"><link rel="prefetch" href="/assets/js/4.3ce47df9.js"><link rel="prefetch" href="/assets/js/40.1f3d0e52.js"><link rel="prefetch" href="/assets/js/41.8e4ee580.js"><link rel="prefetch" href="/assets/js/42.690cbafe.js"><link rel="prefetch" href="/assets/js/43.c4aa2221.js"><link rel="prefetch" href="/assets/js/44.fb378084.js"><link rel="prefetch" href="/assets/js/45.77700693.js"><link rel="prefetch" href="/assets/js/46.d507cb48.js"><link rel="prefetch" href="/assets/js/47.e3a1008a.js"><link rel="prefetch" href="/assets/js/48.d6c9c78e.js"><link rel="prefetch" href="/assets/js/49.4667e7c0.js"><link rel="prefetch" href="/assets/js/5.6d641d4f.js"><link rel="prefetch" href="/assets/js/50.8ec4f4f5.js"><link rel="prefetch" href="/assets/js/51.d9305c51.js"><link rel="prefetch" href="/assets/js/52.db11222b.js"><link rel="prefetch" href="/assets/js/53.61d62d9d.js"><link rel="prefetch" href="/assets/js/54.f20b6c65.js"><link rel="prefetch" href="/assets/js/55.0a4f147e.js"><link rel="prefetch" href="/assets/js/56.cd32d35a.js"><link rel="prefetch" href="/assets/js/57.a5fde1bb.js"><link rel="prefetch" href="/assets/js/58.a0a97490.js"><link rel="prefetch" href="/assets/js/59.1bb4fab1.js"><link rel="prefetch" href="/assets/js/6.5f336290.js"><link rel="prefetch" href="/assets/js/60.e6987c2a.js"><link rel="prefetch" href="/assets/js/61.419fcdc8.js"><link rel="prefetch" href="/assets/js/62.5a9d4e12.js"><link rel="prefetch" href="/assets/js/63.be6ca5ce.js"><link rel="prefetch" href="/assets/js/64.8aef8f94.js"><link rel="prefetch" href="/assets/js/65.16450833.js"><link rel="prefetch" href="/assets/js/66.2969e029.js"><link rel="prefetch" href="/assets/js/67.e3a3c472.js"><link rel="prefetch" href="/assets/js/68.6ff65261.js"><link rel="prefetch" href="/assets/js/69.cf4da88b.js"><link rel="prefetch" href="/assets/js/7.6a1d4e29.js"><link rel="prefetch" href="/assets/js/70.1472620a.js"><link rel="prefetch" href="/assets/js/71.1e7cb0a1.js"><link rel="prefetch" href="/assets/js/72.2bf85313.js"><link rel="prefetch" href="/assets/js/73.d9cb4d96.js"><link rel="prefetch" href="/assets/js/74.0c536977.js"><link rel="prefetch" href="/assets/js/75.4f82c6a8.js"><link rel="prefetch" href="/assets/js/76.42f3f0b7.js"><link rel="prefetch" href="/assets/js/77.70961c41.js"><link rel="prefetch" href="/assets/js/78.2f45f2fc.js"><link rel="prefetch" href="/assets/js/79.a68a9b1c.js"><link rel="prefetch" href="/assets/js/8.378e22b1.js"><link rel="prefetch" href="/assets/js/80.6cb339b8.js"><link rel="prefetch" href="/assets/js/81.5849f124.js"><link rel="prefetch" href="/assets/js/82.f5531d2d.js"><link rel="prefetch" href="/assets/js/83.47badffa.js"><link rel="prefetch" href="/assets/js/84.dece7e94.js"><link rel="prefetch" href="/assets/js/85.d9e7f122.js"><link rel="prefetch" href="/assets/js/86.233d7088.js"><link rel="prefetch" href="/assets/js/87.ce5dff2e.js"><link rel="prefetch" href="/assets/js/88.f976bef7.js"><link rel="prefetch" href="/assets/js/89.8473e540.js"><link rel="prefetch" href="/assets/js/9.9eb4a88c.js"><link rel="prefetch" href="/assets/js/90.d6288b36.js"><link rel="prefetch" href="/assets/js/91.989f4dea.js"><link rel="prefetch" href="/assets/js/92.6ff507f9.js">
    <link rel="stylesheet" href="/assets/css/0.styles.f057962c.css">
  </head>
  <body>
    <div id="app" data-server-rendered="true"><div class="theme-container no-sidebar" data-v-5bb33761><div data-v-5bb33761><div class="password-shadow password-wrapper-out" style="display:none;" data-v-59e6cb88 data-v-5bb33761 data-v-5bb33761><h3 class="title" data-v-59e6cb88>星月前端博客</h3> <p class="description" data-v-59e6cb88>The top of the mountain is not the end, nor is the other mountain.</p> <label id="box" class="inputBox" data-v-59e6cb88><input type="password" value="" data-v-59e6cb88> <span data-v-59e6cb88>Konck! Knock!</span> <button data-v-59e6cb88>OK</button></label> <div class="footer" data-v-59e6cb88><span data-v-59e6cb88><i class="iconfont reco-theme" data-v-59e6cb88></i> <a target="blank" href="https://vuepress-theme-reco.recoluan.com" data-v-59e6cb88>vuePress-theme-reco</a></span> <span data-v-59e6cb88><i class="iconfont reco-copyright" data-v-59e6cb88></i> <a data-v-59e6cb88><span data-v-59e6cb88>星月</span>
          
        <!---->
        2022
      </a></span></div></div> <div class="hide" data-v-5bb33761><header class="navbar" data-v-5bb33761><div class="sidebar-button"><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" role="img" viewBox="0 0 448 512" class="icon"><path fill="currentColor" d="M436 124H12c-6.627 0-12-5.373-12-12V80c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12zm0 160H12c-6.627 0-12-5.373-12-12v-32c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12zm0 160H12c-6.627 0-12-5.373-12-12v-32c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12z"></path></svg></div> <a href="/" class="home-link router-link-active"><img src="/logo.png" alt="星月前端博客" class="logo"> <span class="site-name">星月前端博客</span></a> <div class="links"><div class="color-picker"><a class="color-button"><i class="iconfont reco-color"></i></a> <div class="color-picker-menu" style="display:none;"><div class="mode-options"><h4 class="title">Choose mode</h4> <ul class="color-mode-options"><li class="dark">dark</li><li class="auto active">auto</li><li class="light">light</li></ul></div></div></div> <div class="search-box"><i class="iconfont reco-search"></i> <input aria-label="Search" autocomplete="off" spellcheck="false" value=""> <!----></div> <nav class="nav-links can-hide"><div class="nav-item"><a href="/" class="nav-link"><i class="iconfont reco-home"></i>
  首页
</a></div><div class="nav-item"><div class="dropdown-wrapper"><a class="dropdown-title"><span class="title"><i class="iconfont reco-category"></i>
      博客
    </span> <span class="arrow right"></span></a> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/categories/ES6模块化与异步编程/" class="nav-link"><i class="undefined"></i>
  ES6模块化与异步编程
</a></li><li class="dropdown-item"><!----> <a href="/categories/css/" class="nav-link"><i class="undefined"></i>
  css
</a></li><li class="dropdown-item"><!----> <a href="/categories/TypeScript/" class="nav-link"><i class="undefined"></i>
  TypeScript
</a></li><li class="dropdown-item"><!----> <a href="/categories/html/" class="nav-link"><i class="undefined"></i>
  html
</a></li><li class="dropdown-item"><!----> <a href="/categories/JavaScript/" class="nav-link"><i class="undefined"></i>
  JavaScript
</a></li><li class="dropdown-item"><!----> <a href="/categories/Node.js/" class="nav-link"><i class="undefined"></i>
  Node.js
</a></li><li class="dropdown-item"><!----> <a href="/categories/Vue2/" class="nav-link"><i class="undefined"></i>
  Vue2
</a></li><li class="dropdown-item"><!----> <a href="/categories/Vue3/" class="nav-link"><i class="undefined"></i>
  Vue3
</a></li><li class="dropdown-item"><!----> <a href="/categories/webpack/" class="nav-link"><i class="undefined"></i>
  webpack
</a></li></ul></div></div><div class="nav-item"><a href="/tag/" class="nav-link"><i class="iconfont reco-tag"></i>
  标签
</a></div><div class="nav-item"><a href="/timeline/" class="nav-link"><i class="iconfont reco-date"></i>
  时间线
</a></div><div class="nav-item"><a href="/docs/theme-reco/" class="nav-link"><i class="iconfont reco-message"></i>
  随记
</a></div><div class="nav-item"><div class="dropdown-wrapper"><a class="dropdown-title"><span class="title"><i class="iconfont reco-message"></i>
      联系方式
    </span> <span class="arrow right"></span></a> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="https://github.com/858399075" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont reco-github"></i>
  GitHub
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-item"><!----> <a href="https://gitee.com/xingyueqianduan" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont reco-github"></i>
  Gitee
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-item"><!----> <a href="http://blog.yhxweb.top/" target="_blank" rel="noopener noreferrer" class="nav-link external"><i></i>
  QQ:76120220
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-item"><!----> <a href="http://blog.yhxweb.top/" target="_blank" rel="noopener noreferrer" class="nav-link external"><i></i>
  WX:15237156071
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul></div></div> <!----></nav></div></header> <div class="sidebar-mask" data-v-5bb33761></div> <aside class="sidebar" data-v-5bb33761><div class="personal-info-wrapper" data-v-1fad0c41 data-v-5bb33761><img src="/avatar.png" alt="author-avatar" class="personal-img" data-v-1fad0c41> <h3 class="name" data-v-1fad0c41>
    星月
  </h3> <div class="num" data-v-1fad0c41><div data-v-1fad0c41><h3 data-v-1fad0c41>82</h3> <h6 data-v-1fad0c41>Articles</h6></div> <div data-v-1fad0c41><h3 data-v-1fad0c41>9</h3> <h6 data-v-1fad0c41>Tags</h6></div></div> <ul class="social-links" data-v-1fad0c41></ul> <hr data-v-1fad0c41></div> <nav class="nav-links"><div class="nav-item"><a href="/" class="nav-link"><i class="iconfont reco-home"></i>
  首页
</a></div><div class="nav-item"><div class="dropdown-wrapper"><a class="dropdown-title"><span class="title"><i class="iconfont reco-category"></i>
      博客
    </span> <span class="arrow right"></span></a> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="/categories/ES6模块化与异步编程/" class="nav-link"><i class="undefined"></i>
  ES6模块化与异步编程
</a></li><li class="dropdown-item"><!----> <a href="/categories/css/" class="nav-link"><i class="undefined"></i>
  css
</a></li><li class="dropdown-item"><!----> <a href="/categories/TypeScript/" class="nav-link"><i class="undefined"></i>
  TypeScript
</a></li><li class="dropdown-item"><!----> <a href="/categories/html/" class="nav-link"><i class="undefined"></i>
  html
</a></li><li class="dropdown-item"><!----> <a href="/categories/JavaScript/" class="nav-link"><i class="undefined"></i>
  JavaScript
</a></li><li class="dropdown-item"><!----> <a href="/categories/Node.js/" class="nav-link"><i class="undefined"></i>
  Node.js
</a></li><li class="dropdown-item"><!----> <a href="/categories/Vue2/" class="nav-link"><i class="undefined"></i>
  Vue2
</a></li><li class="dropdown-item"><!----> <a href="/categories/Vue3/" class="nav-link"><i class="undefined"></i>
  Vue3
</a></li><li class="dropdown-item"><!----> <a href="/categories/webpack/" class="nav-link"><i class="undefined"></i>
  webpack
</a></li></ul></div></div><div class="nav-item"><a href="/tag/" class="nav-link"><i class="iconfont reco-tag"></i>
  标签
</a></div><div class="nav-item"><a href="/timeline/" class="nav-link"><i class="iconfont reco-date"></i>
  时间线
</a></div><div class="nav-item"><a href="/docs/theme-reco/" class="nav-link"><i class="iconfont reco-message"></i>
  随记
</a></div><div class="nav-item"><div class="dropdown-wrapper"><a class="dropdown-title"><span class="title"><i class="iconfont reco-message"></i>
      联系方式
    </span> <span class="arrow right"></span></a> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><!----> <a href="https://github.com/858399075" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont reco-github"></i>
  GitHub
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-item"><!----> <a href="https://gitee.com/xingyueqianduan" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont reco-github"></i>
  Gitee
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-item"><!----> <a href="http://blog.yhxweb.top/" target="_blank" rel="noopener noreferrer" class="nav-link external"><i></i>
  QQ:76120220
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-item"><!----> <a href="http://blog.yhxweb.top/" target="_blank" rel="noopener noreferrer" class="nav-link external"><i></i>
  WX:15237156071
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul></div></div> <!----></nav> <!----> </aside> <div class="password-shadow password-wrapper-in" style="display:none;" data-v-59e6cb88 data-v-5bb33761><h3 class="title" data-v-59e6cb88>EventLoop</h3> <!----> <label id="box" class="inputBox" data-v-59e6cb88><input type="password" value="" data-v-59e6cb88> <span data-v-59e6cb88>Konck! Knock!</span> <button data-v-59e6cb88>OK</button></label> <div class="footer" data-v-59e6cb88><span data-v-59e6cb88><i class="iconfont reco-theme" data-v-59e6cb88></i> <a target="blank" href="https://vuepress-theme-reco.recoluan.com" data-v-59e6cb88>vuePress-theme-reco</a></span> <span data-v-59e6cb88><i class="iconfont reco-copyright" data-v-59e6cb88></i> <a data-v-59e6cb88><span data-v-59e6cb88>星月</span>
          
        <!---->
        2022
      </a></span></div></div> <div data-v-5bb33761><div data-v-5bb33761><main class="page"><section style="display:;"><div class="page-title"><h1 class="title">EventLoop</h1> <div data-v-8a445198><i class="iconfont reco-account" data-v-8a445198><span data-v-8a445198>星月</span></i> <i class="iconfont reco-date" data-v-8a445198><span data-v-8a445198>8/7/2022</span></i> <!----> <i class="tags iconfont reco-tag" data-v-8a445198><span class="tag-item" data-v-8a445198>ES6</span></i></div></div> <div class="theme-reco-content content__default"><h1 id="eventloop"><a href="#eventloop" class="header-anchor">#</a> EventLoop</h1> <h2 id="_1-javascript-是单线程的语言"><a href="#_1-javascript-是单线程的语言" class="header-anchor">#</a> <strong>1. JavaScript 是单线程的语言</strong></h2> <p>JavaScript 是一门单线程执行的编程语言。也就是说，同一时间只能做一件事情</p> <p><img src="https://img-blog.csdnimg.cn/c707ff3e41f3458596302c92538aef86.png" alt="image-20220809112638972"></p> <p>单线程执行任务队列的问题：</p> <p>如果前一个任务非常耗时，则后续的任务就不得不一直等待，从而导致程序假死的问题。</p> <h2 id="_2-同步任务和异步任务"><a href="#_2-同步任务和异步任务" class="header-anchor">#</a> 2. 同步任务和异步任务</h2> <p>为了防止某个耗时任务导致程序假死的问题，JavaScript 把待执行的任务分为了两类：</p> <p><strong>① 同步任务（synchronous）</strong></p> <ul><li>又叫做非耗时任务，指的是在主线程上排队执行的那些任务</li> <li>只有前一个任务执行完毕，才能执行后一个任务</li></ul> <p><strong>② 异步任务（asynchronous）</strong></p> <ul><li><p>又叫做耗时任务，异步任务由 JavaScript 委托给宿主环境进行执行</p></li> <li><p>当异步任务执行完成后，会通知 JavaScript 主线程执行异步任务的回调函数</p></li> <li></li></ul> <h2 id="_3-同步任务和异步任务的执行过程"><a href="#_3-同步任务和异步任务的执行过程" class="header-anchor">#</a> 3. 同步任务和异步任务的执行过程</h2> <p><img src="https://img-blog.csdnimg.cn/37662ad9d64545e9a9e6964444aefc49.png" alt="image-20220809124915024"></p> <p>① 同步任务由 JavaScript 主线程次序执行；</p> <p>② 异步任务委托给宿主环境执行；</p> <p>③ 已完成的异步任务对应的回调函数，会被加入到任务队列中等待执行；</p> <p>④ JavaScript 主线程的执行栈被清空后，会读取任务队列中的回调函数，次序执行；</p> <p>⑤ JavaScript 主线程不断重复上面的第 4 步</p> <h2 id="_4-eventloop-的基本概念"><a href="#_4-eventloop-的基本概念" class="header-anchor">#</a> 4. EventLoop 的基本概念</h2> <p><img src="https://img-blog.csdnimg.cn/fd9a32262a574ad88d645826fa5a0ce6.png" alt="image-20220809124915024"></p> <p><strong>JavaScript 主线程从“任务队列”中读取异步任务的回调函数，放到执行栈中依次执行。</strong></p> <p><strong>这个过程是循环不断的，所以整个的这种运行机制又称为 EventLoop（事件循环）。</strong></p> <h2 id="_4-结合-eventloop-分析输出的顺序"><a href="#_4-结合-eventloop-分析输出的顺序" class="header-anchor">#</a> <strong>4. 结合 EventLoop 分析输出的顺序</strong></h2> <div class="language-js line-numbers-mode"><pre class="language-js"><code><span class="token keyword">import</span> thenFs <span class="token keyword">from</span> <span class="token string">'then-fs'</span>

console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'A'</span><span class="token punctuation">)</span>
thenFs<span class="token punctuation">.</span><span class="token function">readFile</span><span class="token punctuation">(</span><span class="token string">'./files/1.txt'</span><span class="token punctuation">,</span><span class="token string">'utf8'</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">then</span><span class="token punctuation">(</span><span class="token parameter">dataStr</span> <span class="token operator">=&gt;</span><span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'B'</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'C'</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">)</span>

console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'D'</span><span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>正确的输出结果：ADCB。其中：</p> <ul><li><p>A 和 D 属于同步任务。会根据代码的先后顺序依次被执行</p></li> <li><p>C 和 B 属于异步任务。它们的回调函数会被加入到任务队列中，等待主线程空闲时再执行</p></li></ul></div></section> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">Last Updated: </span> <span class="time">8/9/2022, 9:35:59 PM</span></div></footer> <!----> <div class="comments-wrapper"><!----></div> <ul class="side-bar sub-sidebar-wrapper" style="width:12rem;" data-v-ac050c62><li class="level-2" data-v-ac050c62><a href="/blogs/ES6%E6%A8%A1%E5%9D%97%E5%8C%96%E4%B8%8E%E5%BC%82%E6%AD%A5%E7%BC%96%E7%A8%8B/4.EventLoop.html#_1-javascript-是单线程的语言" class="sidebar-link reco-side-_1-javascript-是单线程的语言" data-v-ac050c62>1. JavaScript 是单线程的语言</a></li><li class="level-2" data-v-ac050c62><a href="/blogs/ES6%E6%A8%A1%E5%9D%97%E5%8C%96%E4%B8%8E%E5%BC%82%E6%AD%A5%E7%BC%96%E7%A8%8B/4.EventLoop.html#_2-同步任务和异步任务" class="sidebar-link reco-side-_2-同步任务和异步任务" data-v-ac050c62>2. 同步任务和异步任务</a></li><li class="level-2" data-v-ac050c62><a href="/blogs/ES6%E6%A8%A1%E5%9D%97%E5%8C%96%E4%B8%8E%E5%BC%82%E6%AD%A5%E7%BC%96%E7%A8%8B/4.EventLoop.html#_3-同步任务和异步任务的执行过程" class="sidebar-link reco-side-_3-同步任务和异步任务的执行过程" data-v-ac050c62>3. 同步任务和异步任务的执行过程</a></li><li class="level-2" data-v-ac050c62><a href="/blogs/ES6%E6%A8%A1%E5%9D%97%E5%8C%96%E4%B8%8E%E5%BC%82%E6%AD%A5%E7%BC%96%E7%A8%8B/4.EventLoop.html#_4-eventloop-的基本概念" class="sidebar-link reco-side-_4-eventloop-的基本概念" data-v-ac050c62>4. EventLoop 的基本概念</a></li><li class="level-2" data-v-ac050c62><a href="/blogs/ES6%E6%A8%A1%E5%9D%97%E5%8C%96%E4%B8%8E%E5%BC%82%E6%AD%A5%E7%BC%96%E7%A8%8B/4.EventLoop.html#_4-结合-eventloop-分析输出的顺序" class="sidebar-link reco-side-_4-结合-eventloop-分析输出的顺序" data-v-ac050c62>4. 结合 EventLoop 分析输出的顺序</a></li></ul></main></div> <!----></div></div></div></div><div class="global-ui"><div class="back-to-ceiling" style="right:1rem;bottom:6rem;width:2.5rem;height:2.5rem;border-radius:.25rem;line-height:2.5rem;display:none;" data-v-c6073ba8 data-v-c6073ba8><svg t="1574745035067" viewBox="0 0 1024 1024" version="1.1" xmlns="http://www.w3.org/2000/svg" p-id="5404" class="icon" data-v-c6073ba8><path d="M526.60727968 10.90185116a27.675 27.675 0 0 0-29.21455937 0c-131.36607665 82.28402758-218.69155461 228.01873535-218.69155402 394.07834331a462.20625001 462.20625001 0 0 0 5.36959153 69.94390903c1.00431239 6.55289093-0.34802892 13.13561351-3.76865779 18.80351572-32.63518765 54.11355614-51.75690182 118.55860487-51.7569018 187.94566865a371.06718723 371.06718723 0 0 0 11.50484808 91.98906777c6.53300375 25.50556257 41.68394495 28.14064038 52.69160883 4.22606766 17.37162448-37.73630017 42.14135425-72.50938081 72.80769204-103.21549295 2.18761121 3.04276886 4.15646224 6.24463696 6.40373557 9.22774369a1871.4375 1871.4375 0 0 0 140.04691725 5.34970492 1866.36093723 1866.36093723 0 0 0 140.04691723-5.34970492c2.24727335-2.98310674 4.21612437-6.18497483 6.3937923-9.2178004 30.66633723 30.70611158 55.4360664 65.4791928 72.80769147 103.21549355 11.00766384 23.91457269 46.15860503 21.27949489 52.69160879-4.22606768a371.15156223 371.15156223 0 0 0 11.514792-91.99901164c0-69.36717486-19.13165746-133.82216804-51.75690182-187.92578088-3.42062944-5.66790279-4.76302748-12.26056868-3.76865837-18.80351632a462.20625001 462.20625001 0 0 0 5.36959269-69.943909c-0.00994388-166.08943902-87.32547796-311.81420293-218.6915546-394.09823051zM605.93803103 357.87693858a93.93749974 93.93749974 0 1 1-187.89594924 6.1e-7 93.93749974 93.93749974 0 0 1 187.89594924-6.1e-7z" p-id="5405" data-v-c6073ba8></path><path d="M429.50777625 765.63860547C429.50777625 803.39355007 466.44236686 1000.39046097 512.00932183 1000.39046097c45.56695499 0 82.4922232-197.00623328 82.5015456-234.7518555 0-37.75494459-36.9345906-68.35043303-82.4922232-68.34111062-45.57627738-0.00932239-82.52019037 30.59548842-82.51086798 68.34111062z" p-id="5406" data-v-c6073ba8></path></svg></div><div class="kanbanniang" data-v-5775ee02><div class="banniang-container" style="display:;" data-v-5775ee02><div class="messageBox" style="right:68px;bottom:190px;display:none;" data-v-5775ee02>
      欢迎来到 星月前端博客
    </div> <div class="operation" style="right:90px;bottom:40px;display:;" data-v-5775ee02><i class="kbnfont kbn-ban-home ban-home" data-v-5775ee02></i> <i class="kbnfont kbn-ban-message message" data-v-5775ee02></i> <i class="kbnfont kbn-ban-close close" data-v-5775ee02></i> <a target="_blank" href="https://vuepress-theme-reco.recoluan.com/views/plugins/kanbanniang.html" data-v-5775ee02><i class="kbnfont kbn-ban-info info" data-v-5775ee02></i></a> <i class="kbnfont kbn-ban-theme skin" style="display:none;" data-v-5775ee02></i></div> <canvas id="banniang" width="150" height="220" class="live2d" style="right:90px;bottom:-20px;opacity:0.9;" data-v-5775ee02></canvas></div> <div class="showBanNiang" style="display:none;" data-v-5775ee02>
    看板娘
  </div></div><canvas id="vuepress-canvas-cursor"></canvas></div></div>
    <script src="/assets/js/app.b0d6e6c9.js" defer></script><script src="/assets/js/3.3f705cf6.js" defer></script><script src="/assets/js/1.5432b32a.js" defer></script><script src="/assets/js/14.e5bc98a2.js" defer></script>
  </body>
</html>
